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1.
Zircon is one of the most commonly used accessory minerals rich in U and Th for(U-Th)/He dating system. Compared with apatite, zircon has a higher He closure temperature (~190℃), which gives it more advantages in solving the problem of source material and thermal history reconstruction in sedimentary basins. However, the crystals of zircons often have U and Th zoning development, with obvious differences in concentration. Even the standard sample of FCT(Fish Canyon Tuff)zircon which is widely used in (U-Th)/He dating has an average age dispersion of about 10%. In this study, the Alphachron He isotope mass spectrometer is used for laser melting of a batch of single grains of FCT zircon(11 grains)to determine their 4He content. The contents of U and Th of parent isotopes are accurately determined by automatic injection of Agilent 7900 ICP-MS and isotope diluent method. The Th/U ratios of the 10 FCT zircons calculated with (U-Th)/He average age in this paper range from 0.52 to 0.67, which are consistent with the Th/U ratios of 186 reported so far. According to the Th/U ratios of 189 FCT zircons published in the statistical literature, we found that only three of them had high Th/U ratios, namely, 1.12, 1.16 and 1.5, the other 186 FCT zircons(occupy>98%) had a Th/U ratio less than 1. Based on previous results and the 10 Th/U ratios measured in this paper, 196 FCT zircons have a normal Th/U ratio ranging from 0.27 to 1.00, with an average ratio of 0.56(n=196). Excluding one abnormally old age, the(U-Th)/He ages of the remaining FCT zircons in this study range from 26.61 to 31.91Ma, with a weighted mean age of (28.8±3.1)Ma (2SD, n=10), which is consistent with the mean age ((28.3±3.1)Ma, 2σ, n=127) or (28.29±2.6)Ma(2σ external error, 9.3%, n=114)obtained by several other international laboratories. This indicates that the zircon single particle(U-Th)/He dating process established by our laboratory is reliable. For the zircon samples with U, Th banding and concentration differences prevailing, determining the distribution of U, Th elements in the crystal prior to the (U-Th)/He experiment is essential for understanding effects of geometry and elemental zoning on nuclear recoil and diffusion and the interpretation of (U-Th)/He age data.  相似文献   

2.
The (U-Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼1 Ma, chemical fractionation within the 238U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [230Th/238U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He ‘ages’ that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U-Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U-Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.  相似文献   

3.
(U-Th)/He isotopic dating has been developed very quickly in recent years, due to the recognition that the thermal history of rock at low temperature can be effective revealed by such dating method. In particular, He closure temperature in apatite (40~80℃) is very low, so apatite (U-Th)/He ages can reflect the thermal history information of the low-temperature stage, and have a good application prospect in the field of low-temperature thermal chronology. However, because of many influence factors and complicated measurement procedures, the development of apatite He dating in China remains in its early stage. In this study, a measurement procedure was established at the (U-Th)/He dating laboratory of Institute of Geology, China Earthquake Administration. We measured the daughter isotopic helium by diode laser heating four batches of a total seventy-five grains of Durango apatite in an Alphachron helium mass spectrometry system. Then the apatite grains were dissolved to precisely measure the concentration of parent nuclides (U, Th)using the solution isotope dilution method through an automatic sampling ICP-MS (Agilent 7900). Results show that the Th/U values of Durango apatite grains were in the range of 17.23 to 23.60, while all the 75ages were in the range of 28.61 to 34.51Ma with an average of (31.71±1.55)Ma (1σ), which are consistent with the international calibrated ages.  相似文献   

4.
Yu  Shun  Chen  Wen  Sun  Jingbo  Shen  Ze 《中国科学:地球科学(英文版)》2019,62(4):719-732
To gain a better quantitative understanding of zircon(U-Th)/He ages and evaluate the applicability of zircon(U-Th)/He dating, the diffusion characteristics, He diffusion kinetics, helium partial retention zone(HePRZ), closure temperature, and(U-Th)/He ages were investigated using high-precision laboratory step heating experiments based on the thermally activated diffusion process. The ln(D/a~2) in Fish Canyon Tuff(FCT) zircons determined from laboratory step heating experiments was negatively correlated with reciprocal temperature, as expected for thermally activated volume diffusion. The zircon activation energies ranged from 144 to 184 kJ mol~(-1) with a mean of 169±12 kJ mol~(-1). The closure temperatures ranged from 144 to 216°C(a cooling rate of 10°C Ma~(-1) and an effective grain radius of 38–60 μm) with an average of 176±18°C. The calculated closure temperature increased with increasing cooling rate, yielding an average zircon He closure temperature of ～136°C at a slow cooling rate of 0.1°C Ma~(-1), whereas the closure value was ～199°C at a cooling rate of 100°C Ma~(-1). The closure temperature increased with the equivalent spherical radius assuming a constant cooling rate. The He ages from FCT zircons were negligibly affected by grain size because of the rapid cooling. He preserved in the zircon was sensitive to temperature and holding time, and the temperature range for zircon HePRZ gradually decreased with increasing holding time. The(U-Th)/He ages from 26 FCT zircons yielded an algorithmic mean of 28.3±0.3 Ma(S.E.) and a geometric mean of 28.4±0.3 Ma(S.E.), consistent with the ages of 28.4±1.9 Ma reported by other laboratories. The FCT zircons were characterized by rapid cooling, young(U-Th)/He ages with good reproducibility, and low alpha doses. Weak correlations between the He ages and effective uranium(eU) concentrations from the FCT zircons indicated radiation damage did not significantly affect He diffusivity.  相似文献   

5.
Single-crystal (U-Th)/He dating of 32 apatite and zircon crystals from an impact breccia yielded a weighted mean age of 663 ± 28 ka (n = 3; 4.2 % 2σ uncertainties) for the Monturaqui impact structure, Chile. This ~350 m diameter simple crater preserves a small volume of impactite consisting of polymict breccias that are dominated by reworked target rock clasts. The small size, young age and limited availability of melt material for traditional geochronological techniques made Monturaqui a good test to define the lower limits of the (U-Th)/He system to successfully date impact events. Numerical modeling of 4He loss in apatite and zircon crystals shows that, for even small craters such as Monturaqui, the short-lived compressional stage and shock metamorphic stage can account for the observed partial to full resetting of (U-Th)/He ages in accessory minerals. Despite the distinctly different 4He diffusion parameters of apatite and zircon, the 2σ-overlapping youngest ages are recorded in both populations of minerals, which supports the inference that the weighted mean of the youngest (U-Th)/He population is the age of formation of this impact structure.  相似文献   

6.
Apatites from four pairs of samples of Precambrian basement from the western Canadian shield were analyzed by (U–Th)/He thermochronometry to test for the influence of radiation damage on apatite (U–Th)/He dates in this cratonic region. Recent studies have demonstrated that the accumulation of radiation damage increases the apatite He retentivity, so that apatites with a span of effective U concentrations, eU, that experienced the same thermal history may be characterized by a range of closure temperatures. In this investigation, each sample pair consisted of a mafic dike cross-cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eU. The apatites yielded (U–Th)/He dates from 846 to 123 Ma, and were positively correlated with eU within each sample pair. These results can be explained using a model that tracks the evolution of He mobility in response to the accumulation of radiation damage. When coupled with regional geological constraints, the data appear to require partial to complete He loss due to burial and reheating in Phanerozoic time. New apatite fission- track dates and length data were obtained for five of these samples. The apatite fission- track dates are Proterozoic regardless of apatite eU. Thermal history simulations indicate that the apatite fission-track data are compatible with the (U–Th)/He results, although the thermal histories are not identical in detail and the fission-track results alone do not require Phanerozoic heating. Together the data are consistent with burial of this region by ≥ 1 km of Phanerozoic strata that were subsequently denuded, thus pointing toward significant Phanerozoic deposition in the North American cratonic interior hundreds of kilometers east of where previously documented. The results suggest that exploiting radiation damage control on apatite (U–Th)/He dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter restrictions on thermal histories in low relief cratonic landscapes than was previously possible.  相似文献   

7.
Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of 4He in the diamonds proper (liberated by combustion) shows a 1/r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that 4He has been implanted into the diamonds from -decaying elements in the surrounding matrix. The saturation concentration of 4He(5.6 × 10−4 cm3 STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of 238U accompanies the radiogenic 4He; the 136Xef/4He ratio of (2.5 ± 0.3) × 10−9 agrees well with the production ratio of 2.3 × 10−9 expected in a reservoir where Th/U 3.3. Radiogenic 40Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation.

3He, on the other hand is mainly released during pyrolysis and hence is apparently carried by ‘contaminants’. The concentration in the diamonds proper is of the order of 4 × 10−12 cm3 STP/g, with a 3He/4He ratio of 1 × 10−8. Excess 21Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.  相似文献   


8.
Archean komatiitic and tholeiitic lavas from Newton Township, Ontario, have a sufficient range of Sm-Nd ratios to define a well-constrained line on the normal 143Nd/144Nd vs. 147Sm/144Nd isochron plot. The data give an isochron age of 2826 ± 64Ma, and an initial εNdof+2.65 ± 0.26. However, U-Pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite-tholeiite suite give an age of 2697 ± 1.1Ma. There is strong evident that the zircon age is the eruption age, suggesting that the older Sm-Nd age is incorrect and probably results from mixing between isotopically distinct mantle sources. At the time of eruption, the sources had εNd values from about +4.2 to +1.6, indicating that the Archean mantle in this area was markedly heterogeneous and not uniformly depleted.  相似文献   

9.
We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.3He/4He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 RA (RA is the atmospheric ratio of1.39 × 10−6), encompassing the range of previously reported values for MORB erupted away from high3He/4He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in3He/4He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in3He/4He along the ridge can be used to refine the hotspot source-migrating-ridge sink model.

MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by87Sr/86Sr of 0.7022, Pb isotopes close to the geochron and with206Pb/204Pb of 17.7, and3He/4He of 8.6–8.9 RA. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the3He/4He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of3He/4He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pbratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity.

Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.  相似文献   


10.
Noble gas concentrations and isotopic compositions have been determined for four submarine volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin. The samples are the least differentiated ones from this area, and they display enrichments in fluid-mobile elements similar to the nearby island arc. 3He/4He ratios are slightly below average MORB (6.8–7.8 times atmospheric), whereas Ne, Ar, Kr, and Xe have isotopic compositions very similar to air. Together with previously published data from the Valu Fa Ridge and other spreading segments in the Lau Basin, our data show a systematic latitudinal variation of increasing Ne, Ar, Kr, and Xe abundances from north to south as well as Ne and Ar isotopic compositions changing from MORB-like to atmosphere-like in the same direction. Moreover, isotopic compositions and noble gas abundances of the lavas correlate strongly with Ba/Nb ratios and H2O concentrations. Based on these observations and mass balance arguments, we propose that the atmospheric noble gases come from the subducting oceanic crust and are not due to shallow contamination with air dissolved in seawater or assimilation of old crust. Our data suggest that the noble gases released from the subducting slab are atmospheric and thus contain little or no solar He and Ne. In addition to the fact that ratios of He to heavy noble gases are small in aged ocean crust, He has possibly fractionated from the other noble gases due to its higher diffusivity, and thus He transport from the subducting slab into the mantle wedge is probably insignificant. We propose that the 3He/4He ratios lower than MORB observed in the VFR lavas result from radiogenic ingrowth of He in a highly depleted, and hence degassed, mantle wedge after the enrichment of U and Th released from the downgoing slab.  相似文献   

11.
喻顺  陈文  张斌  孙敬博  李超  袁霞  沈泽  杨莉  马勋 《地球物理学报》2016,59(8):2922-2936
天山是中亚造山带重要组成部分,其中-新生代构造热演化及隆升剥露史研究是认识中亚造山带构造变形过程与机制的关键.本文应用磷灰石(U-Th)/He技术重建中天山南缘科克苏河地区中-新生代构造热演化及隆升剥蚀过程.磷灰石(U-Th)/He数据综合解释及热演化史模拟表明该地区至少存在晚白垩世、早中新世、晚中新世3期快速隆升剥蚀事件,起始时间分别为~90Ma、~13Ma及~5Ma,且这3期隆升剥蚀事件在整个天山地区具有广泛的可对比性.相对于磷灰石裂变径迹,磷灰石(U-Th)/He年龄记录了中天山南缘地质演化史中更新和更近的热信息,即中天山在晚中新世(~5 Ma)快速隆升剥蚀,其剥蚀速率为~0.47mm·a~(-1),剥蚀厚度为~2300m.总体上,中天山科克苏地区隆升剥蚀起始时间从天山造山带向昭苏盆地(由南向北)逐渐变老,表明了中天山南缘隆升剥蚀存在不均一性,并发生了多期揭顶剥蚀事件.  相似文献   

12.
Zircon(U-Th)/He thermochronometry was used as an effective indicator to study the thermal history of deep sedimentary basins with high temperatures.Zircon He ages and closure temperatures are very important parameters.In this paper,detrital zircon He closure temperature was studied by establishing the evolutionary pattern between zircon He ages and burial depth of borehole samples obtained from the Cenozoic strata in the Bohai Bay and Tarim basins,which have different thermal settings.The results show that the zircon He closure temperature of natural evolution samples is 200℃,which is higher than the temperature obtained from thermal simulation experiments(183℃).The temperature range of 140-200℃corresponds to the zircon helium partial retention zone.By properly understanding zircon He closure temperatures,zircon(U-Th)/He thermochronometry can provide a correct explanation of the He ages,and significant guidance in the study of the evolution of source rocks and the process of hydrocarbon accumulation in deep sedimentary basins.  相似文献   

13.
(U-Th)/He热定年技术是近年来用于沉积盆地热史研究的新技术,目前主要是利用磷灰石和锆石的He年龄来揭示地层的构造抬升和热历史.本文依据塔里木盆地钻井样品的实测磷灰石和锆石(U-Th)/He年龄数据,初步得出了该地区磷灰石(U-Th)/He年龄的封闭温度为85℃,并建立了深度/温度－年龄演化模式;锆石则未达到其较高的封闭温度.综合利用本次实测的He年龄数据结合磷灰石裂变径迹和等效镜质组反射率等古温标,模拟计算了塔里木盆地孔雀1井（KQ1）自奥陶纪末期以来的热历史.模拟结果表明,孔雀1井区奥陶纪末期的地温梯度可达35.5℃/km,志留纪—泥盆纪时期的地温梯度为33.3～34.5℃/km,白垩纪末期地温梯度27.6℃/km左右.因此,(U-Th)/He年龄结合其他古温标综合模拟的方法可以很好地揭示沉积盆地的热历史.特别是该技术为缺乏常规古温标的塔里木盆地下古生界碳酸盐岩层系所经受热史的恢复提供了新的方法.  相似文献   

14.
Volcanic rocks from subduction zones are widely believed to originate by partial melting of mantle lherzolite modified by the addition of a fluid or melt extracted from the down-going slab. U-series disequilibrium in such magmas is commonly attributed to this particular melting process. A detailed study of U-series isotopes in the 650 y. B.P. eruptive sequence of Mt. Pelée (Martinique) shows that plinian products are in radioactive equilibrium, whereas dome-forming products of the same eruption are characterized by 238U-230Th disequilibrium. The same features apply to other plinian and dome-forming products of this volcano and systematically correspond to different eruptive styles. We attribute these characteristics to variable superficial interaction of magmas with the hydrothermal system during the final stages of eruption rather than to deep magma genesis processes. This conclusion might be generally applicable to arc magmas.  相似文献   

15.
沉积盆地埋藏史和热历史重建是了解盆地成因和油气形成条件的重要依据,而目前利用古温标手段研究珠江口盆地内东沙隆起热史和埋藏史的成果寥寥无几.本研究中,我们基于磷灰石与锆石(U-Th)/He年龄的反演结果给出了更多合理的约束条件,在此基础上对同一套镜质体反射率Ro数据进行了古热流法模拟计算,获得了钻井自中生代以来的地层温度史.反演结果显示LF35-1-1井区在早始新世-早渐新世发生了一期强烈的抬升剥蚀,地层剥蚀量为2000 m左右.拟合获得的埋藏史和热流史显示该井区在早始新世(~55Ma)经历了最高井底古热流(100 mW·m~(-2)),之后热流减小,持续至现今64.3 mW·m~(-2).最高古地温与隆升剥蚀在时间上的耦合体现了抬升事件对地温冷却的影响,但不足以排除基底热流下降因素.本次研究首次将(U-Th)/He技术用于南海北缘深水区盆地的热史研究,获得的热史结果更加符合现有的构造沉积大地构造方面的认识,展示了利用多种古温标手段进行盆地精细热史研究的良好效用.  相似文献   

16.
The uplift and exhumation process in the Tianshan orogen since the late Paleozoic were likely related to the preservation of ore deposits. This study involved reconstructing the whole tectonic thermal history of the Ouxidaban pluton in central South Tianshan Mountains based on hornblende/plagioclase Ar-Ar and zircon/apatite(U-Th)/He methods. The thermal history and uplift process of central South Tianshan Mountains since the late Paleozoic were analyzed according to the results of previous works and cooling/exhumation rate features. The hornblende yields a plateau age of 382.6±3.6 Ma, and the plagioclase yields a weighted mean age of 265.8±4.9 Ma. The Ouxidaban pluton yields weighted mean zircon(U-Th)/He age of 185.8±4.3 Ma and apatite(U-Th)/He age of 31.1±2.9 Ma, respectively. Five stages of tectonic thermal history of South Tianshan Mountains since the late Paleozoic could be discriminated by the cooling curve and modeling simulation:(1) from the latest Silurian to Late Devonian, the average cooling rate of the Ouxidaban pluton was 7.84°C/Ma;(2) from the Late Devonian to the latest Middle Permian, the average cooling rate was about 2.07°C/Ma;(3) from the latest Middle Permian to the middle Eocene, the cooling rate decreased to about 0.68°C/Ma, suggesting that the tectonic activity was gentle at this time;(4) a sudden increase of the cooling rate(5.00°C/Ma) and the exhumation rate(0.17 mm/a), and crustal exhumation of ~1.83 km indicated that the Ouxidaban pluton would suffer a rapid uplift event during the Eocene(~46?35 Ma);(5) since the middle Eocene, the rapid uplift was sustained, and the average cooling rate since then has been 1.14°C/Ma with an exhumation rate of about 0.04 mm/a and an exhumation thickness of 1.33 km. The strong uplift since the Cenozoic would be related to a far-field effect from the Indian and Eurasian plates' collision. However, it was hysteretic that the remote effect was observed in the Tianshan orogenic belt.  相似文献   

17.
柯坪塔格地区位于西南天山与塔里木盆地之间, 是塔里木地台的一部分, 其构造隆升与天山和塔里木盆地的演化密切相关. 本文首次将(U-Th)/He热定年技术应用于该地区构造抬升的研究, 对该区震旦系露头样品的磷灰石和锆石的(U-Th)/He进行了年龄测定和热史模拟, 结果表明柯坪塔格地区主要经历了4期构造抬升事件, 导致震旦系抬升至地表, 其中磷灰石(U-Th)/He年龄揭示了晚白垩世和中新世两期的构造抬升事件. 在早石炭世, 震旦系温度达到最大, 介于133~150°C之间, 结合沉积埋藏史得到当时的最大埋深是3400~3900 m. 在渐新世-中新世, 受印度-欧亚板块碰撞远程效应的影响, 柯坪塔格地区沿柯坪塔格-沙井子断裂向巴楚隆起上逆冲, 地层快速抬升遭受剥蚀. 在15~10 Ma时, 柯坪塔格地区震旦系已抬升至地表. 自早石炭世至今, 柯坪塔格地区总剥蚀量达6170 m. 柯坪塔格地区自中生代以来的构造-热演化史与塔里木盆地北缘是一致的, 但与天山及处于塔里木盆地内部的巴楚隆起的构造抬升过程存在差异. 中新世以后, 受喜山运动远程效应影响, 柯坪塔格和天山才同处于抬升状态; 而巴楚隆起在古近纪早期仍处于抬升剥蚀状态, 与柯坪塔格地区接受沉积相反. 本文利用(U-Th)/He热定年技术成功地揭示了柯坪塔格地区自震旦纪以来的构造-热演化史, 这些结果有利于人们对这一地区构造抬升的正确认识. 同时, 本研究对塔里木盆地的油气勘探及天山地区的构造研究具有指导意义.  相似文献   

18.
Recently, alternative models to estimate the age of diagenetically altered fossil reef corals have been presented based on either redistribution of U or its immediate daughters 234Th and 230Th. Here, we present three methods to estimate the uncertainty of ages derived using an amended version of our coral isochron method [Scholz et al., 2004. U-series dating of diagenetically altered fossil reef corals. Earth and Planetary Science Letters 218, 163–178], which is based on addition/loss of U. The obtained uncertainties are substantially larger than those previously published and should, in general, be more reliable. The isochron method yields larger uncertainties than alternative models based on Th redistribution due to -recoil processes. However, comparison of model open-system ages based on such redistribution of U-series daughters for different sub-samples from an individual coral specimen shows that the smaller errors derived with these models cannot account for the observed variability. We recognise that none of the available models is applicable to all corals, probably reflecting different diagenetic processes even in different sub-samples from one coral specimen. To better understand the diagenetic processes and precisely constrain the uncertainties of the ages derived from diagenetically altered corals, the application of all available models is recommended.  相似文献   

19.
This paper shows how 226Ra–230Th disequilibria can be used to date Holocene volcanic rocks from some well selected volcanoes. A systematic study of these disequilibria on historical or well-dated volcanic samples is indeed first required to test the applicability of this method. Two examples are described here to illustrate its potential. In the case of Mt. Etna, the good correlation observed between (226Ra)0 activities at the time of eruption and Th contents in lava flows from the last two millennia [M. Condomines, J.C. Tanguy, V. Michaud, Magma dynamics at Mt. Etna: constraints from U–Th–Ra–Pb radioactive disequilibria and Sr isotopes in historical lavas, Earth Planet. Sci. Lett. 132 (1995) 25–41] is used to infer the ages of several newly analysed lava flows. The calculated ages are in good agreement with those deduced from the archaeomagnetic curve describing the variation of the geomagnetic field direction in southern Italy [J.C. Tanguy, I. Bucur, J.F.C. Thompson, Geomagnetic secular variation in Sicily and revised ages of historic lavas from Mt. Etna, Nature 318 (1985) 453–455, J.C. Tanguy, M. Le Goff, V. Chillemi, A. Paiotti, C. Principe, S. La Delfa, G. Patane, Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires, C. R. Acad. Sci. Paris 329 (1999) 557–564, J.C. Tanguy, M. Le Goff, C. Principe, S. Arrighi, V. Chillemi, A. Paiotti, S. La Delfa, G. Patane, Archaeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits. Earth Planet. Sci. Lett. 211 (2003) 111–124]. We also present a whole set of new U-series data on historical, recent, and older samples from Merapi (Indonesia), and show that the (226Ra)/Ba ratio has probably maintained a quasi-steady state value during at least the past four millennia, and can be used to infer the (226Ra)0/Ba ratio of old volcanics at the time of eruption, and thus their ages. Comparison with 14C ages available on three samples [R. Gertisser, J. Keller, Temporal variations in magma composition at Merapi volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity, J. Volcanol. Geotherm. Res. 123 (2003) 1–23] shows an excellent agreement. These dating methods, based on the post-eruptive decrease of 226Ra excesses can be confidently used to date young rocks on both volcanoes, an important step to infer their recent eruptive history and magmatic evolution. It also opens the possibility to extend the geomagnetic field variation curve back into the past few millennia. The promising results obtained in this work should encourage new systematic U-series studies to test the applicability of such methods to other permanently active volcanoes showing 226Ra excesses.  相似文献   

20.
本文综合运用磷灰石-锆石裂变径迹和（U-Th）/He、镜质体反射率及盆地模拟等手段,深入细致地探讨了中扬子江汉平原簰洲湾地区中、新生代构造-热史演化过程.研究结果表明,研究区中-新生代大规模构造抬升剥蚀、地层冷却事件始于早白垩世（140-130 Ma）;大规模抬升冷却过程主要发生在早白垩世中后期至晚白垩世.研究区虽然可能存在一定厚度的晚白垩世-古近纪地层沉积,总体沉积规模相对较小.综合分析认为,区内应该存在较大厚度的中侏罗统或/和上侏罗统乃至早白垩世地层的沉积;而现今残存中生代中、上侏罗统地层相对较薄,主要是由于后期持续构造抬升剥蚀造成的,估计总剥蚀厚度约4300 m左右.区内中生代地层在早白垩世达到最大古地温,而不是在古近纪沉积末期;上三叠统地层最大古地温在170~190℃之间.热史分析结果表明,区内古生代古热流相对稳定,平均热流在53.64 mW·m-2;早侏罗世末期古热流开始降低,在早白垩世初期古热流约为48.38 mW·m-2.  相似文献   

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